1. Field of the Invention
This invention relates to integrated circuits and, more particularly, to methods for testing integrated circuits having boundary-scan test circuitry connected in a series-linked boundary test scan chain with other integrated circuits in order to determine the number of programmable integrated circuits connected in the array and the total number of register bits in the chain of boundary-scan test circuits.
2. History of the Prior Art
As integrated circuits have become physically smaller while including more and more individual elements, it has become more difficult to test those circuits. In order to assure that such circuits may be tested accurately without inordinate expense, these integrated circuits have been equipped with circuitry adapted to allow boundary-scan testing. Boundary-scan testing uses a plurality of shift register stages built into each integrated circuit. Boundary-scan controller circuitry in each integrated circuit controls the transfer of data serially from an input port to an output port through the stages of the boundary-scan shift register and allows use of the data so that circuit testing may be conducted from external terminals without the need for probes and other imprecise instruments. Boundary-scan testing makes the test process for integrated circuits so equipped faster and more accurate.
An industry standard has been implemented for boundary-scan test operations which allows the boundary-scan test circuitry of a plurality of integrated circuits from different manufacturers to be connected in a serial chain within an electronic system. This standard is described in an industry specification, IEEE JTAG 1149.1 ("the Standard"). The Standard provides a protocol by which various test functions may be accomplished through specified test ports defined by the specification. Essentially, the Standard outlines the details of the serial path of linked test registers (called a boundary-scan register chain) through the integrated circuits and defines the controllers in each integrated circuit. The linked serial path of the boundary-scan register chain allows data to be transferred to various test and other registers within any of the integrated circuits. From these registers, various operations may be conducted by the controllers with the specific integrated circuits.
One problem that has arisen for manufacturers in implementing the testing has been the need to know the length of the boundary-scan chain and the number of individual integrated circuits in that chain. Unless these facts are known, it is impossible to provide instructions and data to the desired boundary-scan circuitry to correctly test any specific integrated circuit. To date, that knowledge has had to be furnished by the manufacturer of the system before boundary-scan testing of a particular integrated circuit could be conducted.